This invention relates to a toner for developing an electrostatic image in electrophotography and electrostatic recording.
The development processes for the electrostatic latent image are broadly classified into liquid development process and dry development process. The present invention relates to a toner for use in dry development process. Further, as for fixing the developed toner image on a support such as paper, there have been known methods of fixing by application of heat such as hot roll fixing, flashing, high frequency heating, and oven heating, by application of pressure by means of rolls, and by using an organic solvent. This invention relates to a toner particularly suitable for hot roll fixing.
Recent electrophotographic copiers generally adopt hot roll fixing for the reasons of high thermal efficiency, excellent fixing performance, good image quality, and compactness of the equipment. It is popular to use the hot roll fixing particularly in high speed fixing system. In the hot roll fixing, however, upon being heated, the toner becomes soft and tacky and a portion of it adheres to the hot roll surface, giving rise to a phenomenon called offset.
To keep the toner from offset various equipments and methods for fixing have heretofore been proposed. There is used, for example, a fixing equipment utilizing hot roll, the surface layer of which is made of a material having an excellent release property and, in addition, the surface layer is overcoated with an anti-offset liquid such as a silicone oil. Although efficient in preventing the offset phenomenon from occurring, such an equipment has disadvantages in that a disagreeable odor is emitted owing to degeneration of the anti-offset liquid, that the equipment is of a large size to accommodate the requisite feeding device for the anti-offset liquid and that an increase in the cost is resulted from the complicated mechanism of the equipment and the higher accuracy of the equipment required for the steady operation. Therefore, there has been made an effort to develop a method of preventing the offset phenomenon, requiring entirely no or only a small amount of anti-offset liquid which affects the cost insignificantly. For example, Japanese Patent Application "Kokai" (Laid-open) No. 65,231/74 discloses that the offset can be eliminated by incorporating a small amount of a low molecular weight polypropylene into the toner composition.
Regarding the binder for the toner, various attempts have also been made. For instance, it is disclosed in U.S. Pat. No. 3,941,878 that a polymer capable of crosslinking is used as binder and in U.S. Pat. Nos. 4,386,147 and 4,486,524 that a resin having a broad molecular weight distribution is used as binder. There are also described in Japanese Patent Application "Kokai" (Laid-open) Nos. 68,752/83 and 102,246/83 that a modified epoxy resin having a vinyl group and a vinyl-type polymer which is graft-copolymerized with a polyester resin having a vinyl group are used as binders, respectively.
However, when the low molecular weight polypropylene is used as an anti-offset agent, it is desirable to use the polymer in a least possible amount, because otherwise the toner tends to become non-uniform and cause dirty background or dusting. When the polymer capable of crosslinking and the graft copolymer are used as binder, a general tendency is a decline in fixing properties and such a tendency becomes more marked with the increase in fixing speed. When the polymer having a broad molecular weight distribution is used, although it is possible to prevent the offset phenomenon without injuring the fixing properties, a problem encountered is an economic disadvantage, because it is generally not easy to prepare such a resin.